Weed hosts of Ralstonia solanacearum in Taiwan.
Weed plants were randomly collected from fields in Taiwan and their roots were assayed for the presence of R. solanacearum. The fields were randomly selected where no host crops were growing or no diseased crop plants were observed. A total of 1844 weed plants belonging to 78 species in 23 families were collected from 73 fields at various localities of Taiwan. R. solanacearum was only isolated from the roots of 21 plants representing nine species: Ageratum conyzoides, Ageratum houstonianum, Bidens bipinnata, Crassocephalum crepidioides, Cyperus rotundus, Drymaria cordata, Euphorbia hirta, Kyllinga brevifolia and Ludwigia octovalvis. All of these plants were symptomless carriers, and were found only in a few fields. The rates of positive isolation from these weeds were generally low, ranging from 2.2 to 6.6%, with the exceptions of D. cordata (25%) and K. brevifolia (14.2%). All strains of R. solanacearum from weeds were classified as race 1 and biovar 3, and their phenotypic characteristics were similar to those strains from crops. Fourteen common weeds tested including the above symptomless carriers varied in their susceptibility to R. solanacearum when artificially inoculated by stem-prick and soil-drenching methods. D. cordata, C. crepidioides, C. rotundus, K. brevifolia, Portulaca oleracea and Solanum nigrum were the most susceptible, showing rapid wilting after stem inoculation with most of strains (16 strains tested), while E. hirta, L. octovalvis, Vernonia cinerea and Phyllanthus urinaria were most resistant, exhibiting no symptoms in all on most of the inoculated plants. In addition, Physalis angulata was highly susceptible to some strains but was highly resistant to others. R. solanacearum was detected in the roots and stems of nearly all or some of the symptomless resistant weed plants except in those of E. hirta and L. octovalvis when they were inoculated by the soil-drenching method. R. solanacearum multiplied and its population increased rapidly in the roots and stems of susceptible weeds after planting in infested soil, and high population levels were also maintained in the rhizospheres for five weeks. The results indicate that symptomless weeds that carried R. solanacearum occur in some fields not known to be infested, and that the susceptible weeds and the symptomless weeds that could be systemically infected are potential hosts of R. solanacearum in Taiwan.